Embodiments of the present invention relate to an image processing device of a computer tomography system, to a computer tomography system and to a method for determining calibration data for a computer tomography system.
In contrast to standard x-ray systems, computer tomography systems (CTs) offer the advantage that cross-section images may be computed both two-dimensionally and also three-dimensionally by means of a computer. This calculation is based on a plurality of digitized x-ray recordings which represent an object from different radiation angles and may be combined taking geometrical parameters of the CT into account. By means of an exact knowledge of such geometrical parameters or generally of the recording geometry, both in two-dimensional and also in three-dimensional computer tomography an artifact-free reconstruction of the sectional recordings may be realized.
FIG. 3a shows a schematical setup of a computer tomography system in a cone beam geometry, while FIG. 3b shows a spiral computer tomography system which is also based on cone beam geometry.
The computer tomography system illustrated in FIG. 3a comprises a recording unit 10 having a radiation source 12, like e.g. an x-ray tube, and a radiation detector 14 associated with the radiation source. Between the radiation source 12 and the detector 14, the object 16 to be x-rayed is rotatably arranged. The object 16 may be rotated for example by means of a manipulation means around a rotary center or an axis of rotation 17. The axis of rotation 17 is approximately perpendicular (or possibly slightly tilted) with respect to an x-ray direction 18, which extends from the radiation source 12 to the radiation detector 14. This x-ray direction 18 is centered within the radiation cone 20 and passes in parallel to the central beam which is defined as a beam between the point of x-ray generation, i.e. a focal spot of the radiation source 12, and the center of the detector 14.
Radiation spreads along this beam cone, e.g. x-radiation of the radiation source 12, and x-rays the object 16. The absorption of the radiation by the object 16 may be detected by means of the radiation detector 14 in the form of an x-ray recording 16′. Here, the detector 14 transforms the received radiation of the radiation source 12 which is weakened by the object 16 into grey values which represent a measure for weakening. During the rotation a plurality of x-ray recordings 16′ (projection) is recorded from different recording angles on the basis of which a computer tomography recording may be reconstructed.
This functioning may also be transferred to spiral or helix computer tomography systems as it is illustrated in FIG. 3b. A spiral computer tomography system basically comprises the same setup as the computer tomography system of FIG. 3a, wherein, however, the object 16 to be examined may for example be moved by means of the manipulation means itself or an additional lifting means, translationally along the axis of rotation 17, in addition to rotation. With respect to the setup illustrated in FIG. 3b it is possible due to the additional degree of freedom or the translational movement along the axis of rotation 17 to record different tomography or layer recordings of the object 16 from different recording angles in different horizontal planes. Thus, the rotary movement around the axis of rotation 17 is overlaid with the translational movement along the axis of rotation 17, so that the object 16 is rotated several times around the axis of rotation 17 (in connection with a vertical feed) and may thus be x-rayed spirally. The acquired spatial resolution and image sharpness depends on the geometric characteristics of the imaging system.
Using a plurality of x-ray recordings of the x-rayed object 16′ from different recording angles and possibly in several layers or planes, a computer tomography recording is reconstructable, wherein the reconstruction is based on an exact knowledge of the geometrical parameters, like e.g. the relative position of the axis of rotation 17 with respect to the radiation source 12 or a possible tilting (e.g. in a range from 0° to 90°) of the axis of rotation 17. For a sufficient image quality in reconstruction it is assumed that the position of the axis of rotation 17 is known with an accuracy of ±0.2×deff wherein deff is the effective pixel size in the image. The effective pixel size deff is the quotient of the distance of the detector elements dDet with respect to each other and the magnification factor M in the image: deff=dDet/M.
If the relative position (i.e. not necessarily the absolute position) of the axis of rotation 17 is not sufficiently known, this has a negative effect in reconstruction onto the image quality. The image quality further depends on the position of the focal spot, wherein the same typically changes with varying x-ray parameters, like e.g. an x-ray voltage. The position of the rotary axis 17 is thus advantageously determined again for each computer tomography recording (data acquisition), but at least with changed operation parameters. In industrially used computer tomography—in contrast to medical computer tomography which basically deals with an unchanging task—there is also the fact that requirements, for example with respect to material to be examined or with respect to image quality, frequently vary. Consequently, system geometry and the associated geometrical parameters frequently change, so that for example a renewed determination of the position of the axis of rotation 17 or the rotation center is needed.
As it is described in patent document DE 4325351 C2, in known methods for determining the center of rotation a reference object is used. Here, for example a longitudinal object, like e.g. a rod, a needle or a wire is moved to different positions as a reference object by means of a manipulation unit and the positional data of the reference object are recorded. On the basis of the positional data geometrical parameters, like e.g. the relative position of the axis of rotation 17 with respect to the central beam (see x-ray direction 18) and the magnification factor M may be determined. Further, apart from the axis of rotation 17 the translational axis in the overall system may be determined when the reference object is moved in translation. In addition, the method offers the possibility to output a quality factor which forms a measure for the reliability of the calculation of the center of rotation. An alternative method for which for example a translational object movement in the central plane is needed determines the geometrical data by means of imaging a spherical reference object which is moved via a lift axis.
After a successful determination of the axis of rotation 17 or the rotary center, the projection data set of any object to be recorded which is recorded at the same axis position and using the same x-ray parameters may be reconstructed artifact-free. This applies in particular to CTs based on fan-beam geometry or cone beam geometry. However, with the above described methods, the error sources explained in the following may not be ruled out completely which may possibly lead to a decrease of precision in imaging.
Errors for example result by the reference object being exchanged by the object to be imaged 16 before the actual recording but after calibration. By this, the measurement environment, like e.g. ambient temperature, is changed by the exchange. Further, the rotary center 17 may be shifted by the exchange of the object 16 wherein here given a corresponding magnification already a shifting by several micrometers is sufficient to affect image quality. One possibility to minimize this is to attach the reference object for example above the object to be examined. After determining the axis of rotation 17 the reference object may for example be removed via a lift axis from the image area or the reconstruction area may be restricted vertically to the object 16 to be examined.
As an alternative to the above-described method it is also possible to determine geometrical parameters like e.g. the axis of rotation 17 or the center of rotation by means of a pre-scan before data acquisition for the computer tomography recording at the object itself or after data acquisition. In patent document DE 1020070081178 A1, by means of a pre-scan the proportion of scattered rays is determined depending on the angle and introduced into the computer tomography measurement. The proportion of scattered rays serves for calibrating the medical CT, so that the dose may be modulated depending on the angle, as for different x-ray directions through the patient different wavelengths results. In patent document U.S. Pat. No. 5,457,724 A a further pre-scan is described by means of which the position of the patient and its center are determined. In further methods by means of such a pre-scan the position of the patient is determined with the help of contrast agents. All of these methods which are based on a pre-scan need additional time and interaction of the operator with the CT.